Storage cathode ray tube



Dec. 8, 1959 D. w. DAVlS STORAGE CATHODE RAY TUBE Filed Aug. 20, 1958owl mmvrox. Bean M Dar/.

Atteneys United States Patent 2,916,661 STORAGE CATHODE RAY TUBE Dean W.Davis, Fort Wayne, Ind., assignor to International Telephone andTelegraph Corporation I Application August 20, 1958, Serial No. 756,155

8 Claims. (Cl. 315-11) This invention relates generally to storagecathode ray 2,916,661 P atented Dec. 8, 1959 age cathode ray tubes'isthe relative locations of the'high velocity writing beam electron gun,and the low'velocity flood gun. It has commonly been found mostdesirable to mount the flood electron gun along the axis of the tube inorder to avoid image shading in the visual display on the displayscreen. Paradoxically, however, the most tubes and more particularly tostorage cathode ray tubes of thesignal-to-image type in which anelectrical input signal is written into the storage system of the tubeand thereafter read-out as a visual display.

' Conventional signal-to-image storage cathode ray tubes comprise anenclosing envelope with a phosphor display screen arranged at one endand with a conventional electron gun assembly arranged at the other end.A charge storage screen, commonly formed as a fine mesh metal screenwithsecondary ernissive insulator material on one side thereof isdisposed between the display screen and the electron gun with itsinsulator surface facing the electron gun. The electrical input signalis commonly connected to a control grid of the electron gun which isarranged to scan a high velocity electron beam over the insulatorsurface of the storage screen; impingement of the electron beam on theinsulator surface of the charge storage screen results in secondaryemission therefrom thus forming a charge pattern or image on theinsulator surface corresponding to the electrical input signal. Thecharge image which has thus been written on the insulator surface of thestorage screen is commonly fread-out by subsequently flooding the chargestorage screen with a uniform flood beam of low velocity electrons,these low velocity electrons passing through the openings in the chargestorage screen, being modulated by the incremental charges on theinsulator surface thereof, and impinge upon the phosphor display screenthereby forming a visual image corresponding to the charge image on theinsulator surface of the storage screen; the low velocity flood beam ofelectrons is'commonly provided by another conventional electron gun inthe envelope.

In prior signal-to-image storage tubes known to the present applicant,the minimum spot size of the incremental elements of the visual displayis limited to something greater than the size of the mesh openings inthe storage screen because the mesh-formed low velocity electron beamsspread after going through a cross-over close to the storage screen. Itis possible to focus the low,velocity electron beams between the storagescreen and the phosphor display screen with a magnetic focusing fieldthereby to reduce the minimum spot size of the display image and toprovide accompanying increase in'the resolution thereof. However, to thebest of the present applicants knowledge, no practical method has beenfound to restrict the magnetic focusing field to the region between thestorage screen and' the phosphor display screen. In conventionalsignal-to-image tubes, the low velocity flood electron beam is emittedfrom a small area cathode and the effect of the magnetic focusing fieldwhich unavoidably enters the section of the tube containing the floodelectron gun is to focus the low velocity electron beam from the cathodeonto the storage screen instead of allowing the flood beam to spread soas to illuminate the entire area of the insulator surface of the storagescreen as is required in tubes of this type.

desirable place for the high velocity writing electron gun is also alongthe axis of the tube in order to avoid distortion of the charge imageonthe insulator surface of the storage screen. It has, however, been thegeneral practice to mount the writing gun so that it is off-set from theaxis of the tube.

Conventional signal-to-image storage tubes provide a visual displayedimage having high brightness and high contrast capabilities. However, intubes of this type known to the present applicant, the resolution of thedisplayed image deteriorates with increased brightness since, as theflood beam current is increased, the spot size in the displayed imagealso increases. It has therefore long been felt desirable to provide asignal-to-image storage tube in whichv the high contrastcapabilities ofpresent tubes are retained and in which the low velocity electron beamforming the display image could be properly focused thereby to improvethe resolution of the tube. It has further long been felt desirable toprovide a signal-toimage storage tube construction in which the writingbeam gun could be located on the axis of the tube thereby eliminatingthe distortion encountered with the writing gun off-set from the tubeaxis, but in which corresponding off-setting of the source of floodelectrons was not necessary; while the use of ring-type flood gunsconcentrically surrounding the writing gun has been proposed, difficultproblems in gun geometry are found in such constructions in order toinsure uniform coverage of the charge storage screen with the floodbeam.

The storage image converter tube is an image-to-image tube well known inthe art. In such tubes, a large area photocathode is arranged at one endof the tube with a phosphor display screen being arranged at the otherend and a storage screen being arranged intermediate the photocathodeand the display screen. Thus, a radiation image impinged upon thephotocathode excites the same to emit photoelectrons in a patterncorresponding to the incident radiation image, these photoelectronsagain by secondary emission forming a charge image on the insulatorsurface of the storage screen. This charge image in the case of thestorage image converter tube is conventionally read-outv by uniformlyilluminating the photocathode with a source of flood illumination thusexciting the photocathode to emit a uniform beam of low velocityelectrons which, as in the case of the signal-toimage storagetube, passthrough the storage screen, being modulated by the incremental chargesthereon to provide a visual display on the display screen. In thestorage image converter tube, the'. flooding photocathode is, itself, aslarge as theare'a it must flood on the insulator surface of the storagescreen and thus, magnetic focusing of the flood beamimay be providedsince the magnetic field does not, in this case, reduce thefdiameter ofthe flooded phosphor display screen and a planar target electrode spacedfrom the display screen and including a phosphor layer and a layer of'photocathode material facing the display screen and arrangedn o beexcited to emit photoelectrons by fluorescence of the phosphor layer.The writing electron gun is arranged to an electron beam over thephosphor layer of the target electrode thereby causing fluorescence ofthe same and in turn exciting the photocathode materiall ayer to emitphotoelectsons toward the display screen. A perforate charge storagescreen is arranged intermediate the target electrode and thedisplayscreen. Means are provided for impressing an input signal on theelectron gun and .-a suitable potential on the photocathode materiallayer so that scanning of t e P Q Ph layer w t the sls tmnb re n i [tthe input signal causes the photocathode material layer to emitelectrons which are then accelerated to high velocity to produce thecharge infl age on the'storage screen .by secondary emission. Means arefurther provided for exciting the photoeathode material layer to emit auniform flood beam of electronswhich pass through the openings in thestoragescreen with low velocity, being modulated by the chargeimagethereon, thereby to form the visual image on the display screen. Inaccordance with my invention, magnetic focusing means is arranged tofocus the photoelectrons from the photocathode ma terial layer onto thecharge storage screen, and to focus an electron image of the chargestorage screen on to the display screen, the magnetic field provided bythe magnetic focusing meanshaving negligible deleterious effect on theelectron beam from theelectron gun scanning the phosphor layer since themagnetic field is weak in that region of the tube and the velocity ofthe gun electron beam is very high. In one embodiment of my invention,the scanned electron beam from the electron gunis used not only forwriting the input signal upon the phosphor layer in turn exciting thephotocathode to emit electrons accelerated at high velocity toward thestorage screen, but is also used for scanning a uniform intensity beamover the phosphor layer with the potential impressed on the photocathodematerial layer being altered so that it emits a uniform beam ofelectrons accelerated with low velocity toward the storage screen forreading out the charge image stored thereon. In another embodiment of myinvention, the low velocity 'flood photoelectrons from the photocathodematerial layer are provided by illuminating the phosphor layer withflooding source of light positioned outside of the tube envelope. Themagnetic focusing means in one embodiment of my invention comprises anelongated annular coil surrounding the tube envelope and extendingbetween the target electrode and the display screen. This focusing coilprovides an axially polarized magnetic 'field with its flux linesextending parallel to the axis of the, tube in the image section thereofthus effectively focusing the photoelectrons emitted from thephotocathode material layer onto the storage screen and the displayscreen.

i -It is accordingly an object of my invention to provide an improvedstorage cathode ray tube.

Another object of my'invention is to provide an improved signal-to imagestorage cathode ray tube.

A further object of my invention is to provide an improvedsignal-to-image storagecathode ray tube in which the resolutionis'improved.

A still further object of my invention is to provide an improvedsignal-to-image storage cathode ray tube in which the writing electrongun is located along the axis of the tube thereby, elimina n distortionof the written image found in prior tubes having oif-set writing guns.

The above-mentioned and other features and objects of invention andtheplanner of attaining them a will m r ana ram. n the vention i e w b hewaer taq b rr fsr a t thert low as des ripen ontak n con- Walnutsto-im-age storage cathode ray tube incorporating one embodiment of myinvention, and

Fig. 2 is a fragmentary schematic cross-sectional view illustratinganother embodiment of my invention.

Referring now to Fig. 1, my improved signal-to-image storage cathode raytube, generally identified as 1, comprises an enclosing envelope2 havinga conventional phosphor display screen 3 formed at one end thereof. Aconventional high velocity writing electron gun assembly 4 is positionedwithin elongated neck 5 of envelope 2 and comprises a suitable cathodea. control grid 7 and conventional beam forming and acceleratingelements 8, 9 and 10, as is well known in the ,Suitable potentials areapplied to the accelerating and beam forming elements 8, 9 and '10 bymeans of external leads 11 and 12 and the high velocity electron writingbeam 13 produced by the electron gun assembly 4 is scanned in thedesired pattern by means of horizontal and vertical electrostaticdeflection plates 14 and 15 connected rejspectively to external leads.16 and 17; it will be readily understood that while electrostaticdeflection elements 14 and 15 are shown, conventional magneticdeflection of electron beam 13 may be equally advantageously employed.Cathode 6 of electron gun assembly 4 is connected to a suitable sourceof cathode potential -E by external lead 13 while the control grid7 isconnected to switch 19 by means of external lead 20, Switch 19 has afirst position 21 connected to a suitable source of control grid biasingpotential E by means of suitable resistor 22 and also to a souroe ofelectrical input signals by means of coupling capacitor 23. Switch 19has a second position 24 connected to external lead 18 of cathode 6, asshown. A conventional conductive oa n 25 i a ned on eimz r wa w enve o e1 forwardly of electron gun assembly deflection elements 14 and 15,conductive coating 25 being connected to a suitable source of potentialby external lead 26 and thus serves to accelerate electron beam 13, asis well known in the In accordance with my invention, I provide a targetelectrode assembly 27 within envelope 2 and spacedfrom display screen 3,target electron assembly comprising a phosphor layer 28 supported on athin transparent film 29 which in turn supports a layer 30 ofphotocathode material. It will now be seen that electron beam 13 iscaused to scan the phosphor layer 28 by means of deflection elements 14and 15; impingement of electron beam 13 on the phosphor layer 28 causesit to fluoresce in turn exciting the photocathode material layer 30causing it to emit photoelectrons toward display screen 3.

The phosphor layer 28 and photocathode material layer 30 are preferablyconnected together by means of external leads 31 and 32 and in turnconnected to switch'33. Switch 33, which may be ganged with switch 19,as shown by the dashed line 34, also has two positions, its firstposition 35 connecting phosphor layer 218 and photocathode materiallayer 30 to a suitable source of external pot n al, such a Q voltsby exn ea 6, an its second position 37 connecting phosphor layer 28 andphotocathode material layer 30 to another suitable source of externalpotential, such as ground 39, as shown.

A charge storage screen 40 is provided interposed be tween the targetelectrode 27 and display screen ,3, storage screen 40 preferablycomprising a fine mesh metal screen 41 having secondary emissiveinsulator material 42 coated on its surface facing the photocathodematerial layer 30. Fine mesh metal screen has in external iead 43connected to switch 44 whichlin turn has two positions 45 and 46adaptedrespectively to connect the fine mesh metal screen 41 to twoappropriate potentials, such as +15 .volis +18 voits respectively, asshown. A f ne mesh neial secondary electron collector screen .47 isprovidedclosely spaced from the insulator surface 42, of the storagescreen 40 and is connected to a suitable source atrqtsiat a m means atext rna lead 48- R n e e trodes 49, S0 and51 are provided between thetarget electrade 27 and collector screen 47 for establishing a uniformaccelerating field for the photoelectrons emitted from photocathodematerial layer 30, ring electrodes 49, 50 and 51 being connected toappropriate progressively higher voltagesrby means of external leads 52,53 and 54. Suitable n'ng electrodes 55 and 56 for the same purpose aredisposed between storage screen 40 and display screen 3, being againconnected to suitable progressively higher voltages by external leads 57and 58.

In order to focus the photoelectrons emitted from photocathode materiallayer 30 'onto thestorage screen 40 and also onto the display screen 3,a suitable focusing coli 60 is provided surrounding envelope 2 andextending between the target electrode 27 and display screen 3',focusing coil 60 being adapted to be'connected to a suitable source ofenergizing potential by means of external leads 61.

In: operation and during writing of an input signal, switches 19 and 33are in their first positions 21 and 35 respectively connecting controlgrid'7 of electron gun 4 to the source of input signals and to anappropriate grid bias potential and connecting phosphor material layer28 and photocathode material layer 30 to a source of potential highlynegative with respect to the potential impressed on the fine mesh metalscreen 41'of the storage screen 40, i.e., +15 volts, with switch 44 inits position 45. The writing beam 13 from electron gun 4 is thus causedto scan the phosphor layer 28, the intensity of beam 13 being variedresponsive to the input signal applied to control grid 7 of electron gun4 by coupling capacitor 23. The spot caused by impingement of electronbeam 13 on the phosphorvlayer 28 causes corresponding fluorescence ofthe phosphor layer, the light from this fluorescence being transmittedthrough the transparent film 29 thus exciting the photocathode materiallayer 30 to emit photoelectrons toward the storage screen 40. Thephotoelectron image emitted from the photocathode material layer 30 isfocused onto the insulator layer 42 of the storage screen 40 by themagnetic field established by focusing coil 60, impingement of thephotoelectrons on the insulator layer 42 producing secondary electronswhich are collected by the secondary emission collector screen 47. T Acharge pattern or image is thus formed on the insulator layer 42 ofstorage screen 40 proportional to the density of the photoelectron imageand thus in turn corresponding to the electrical signal impressed oncontrol grid 7 of electron gun 4; the charge image written onto theinsulator layer 42 of storage screen 40 cannot easily leak off since thelayer 42'is a good insulator.

In order to read-out the stored image on the insulator layer 42 ofstorage screen 40, switches 19 and 33 are moved to their secondpositions 24 and 37 respectively disconnecting the input signal sourceand control grid biasing voltage from the control grid 7 and connectingit instead to the cathode 6, and impressing a potential, i.e., ground onphosphor layer 28 and photocathode material layer 30 which is onlyslightly negative with respect to the potential impressed on the finemesh metal screen 41 of storage screen 40, i.e., +15 volts. The electronbeam 13 continues to scan the phosphor layer 28, now having uniformintensity by virtue of connection of the control grid '7 to cathode 6.The resulting fluorescence of the phosphor layer 28 caused byimpingement of the uniform intensity electron beam 13 thereon excitesthe photocathode material layer 30 to emit photoelectrons, however, byvirtue of its only slightly negative potential with respect to thestorage screen 40, the photoelectrons now emitted from the photocathodematerial layer 30 now approach the insulator layer 42 with very lowenergy. A uniform flood beam of photoelectrons is thus provided from thephotocathode material layer 30 during the read-out operation, these lowvelocity photoelectrons passing through the openings'in the storagescreen 40, being modulated responsive to the incremental charges on theinsulator surface 42. Thelow velocity photoelectrons which pass'throughthe openings in the storage screen 40 are focused onto the phosphordisplay screen 3 by' the focusing coil 60, the resulting light image onthe display screen 3 being a replica of the charge image whichwas'formed on the insulator layer 42.

The charge image on the insulator layerv 42 may be obliterated, i.e.,erased, by momentarily increasing the potential of the entire surface bya few volts. This is accomplished by switching the potential of the finemesh screen 41 of storage screen 40 by means of switch 44 from itsoriginal potential of +15 volts to a slightly higher potential of +18volts; With switches 19 and 33 still in their second positions, i.e.,establishing the circuit connections for read-out with the signal inputcircuit and source of control grid bias potential removed from controlgrid 7 of electron gun 4 and with the phosphor layer 28 and photocathodematerial layer 30 at ground potential, the photoelectron beam emitted bythe photocathode layer 30 will new approach the insulator surface 42with slightly higher energy occasioned by the increase of the potentialof the fine mesh metal screen 41 from +15 volts to +18 volts. Thephotoelectrons which do not pass through the openings in the storagescreen 40 will strike the insulator surface 42 and will charge it in thenegative sense since the secondary emission will be negligibly small forthe low energy photoelectrons. This negative charge obliterates theformer positive charge returning the insulator surface 42 to itsoriginal potential in readiness for receiving a new charge in the nextwriting operation by returning switches 19 and 33 to their originalpositions 21 and 35 and switch 44 to its original position 45.

With the phosphor layer 28 and photocathode material layer 30 connectedtogether by external leads 31 and 32, as shown, when the potentialapplied thereto is increased by moving switch 33 from its first position35 to its second position 37, the electron beam 13 will no longer befocused on the phosphor layer 28. This fact is, however, of noconsequence, and may even be desirable since the purpose of the electronbeam during the read-out operation is to excite the phosphor layer 28uniformly over its entire area thereby uniformly to excite thephotocathode material layer 30 to provide a uniform low velocity beam ofphotoelectrons during viewing and erasing.

Turning now to Fig. 2 in which like elements are illustrated by likereference numerals, the uniform low velocity flood beam electrons fromthe photocathode material layer 30 may be obtained by flooding the layer30 with light. Thus, in the illustrated embodiment, electric lamps 63and 64 are provided outside of the envelope 2 and arranged to illuminatethe surface of the transparent phosphor layer 28; this requires that thewall section 65 of envelope 2 be transparent, however, this is readilyac complished by means of the use of a transparent conductive coating 25as is well known in the art. Lamps 63 and 64 are energized through leads66 and 67 connected respectively to a suitable source of power by meansof switch 68. Switch 68 may be ganged with switches 19 and 33 so that itis closed in the second positions of switches 19 and 33. Thus, duringread-out and erasing, when switches 19 and 33 are in their secondpositions, as hereinabove described, switch 68 will be closed thusilluminating lamps 63 and 64 in turn uniformly illuminating photocathodematerial layer 30 through transparent phosphor layer 28, uniformly andinstantaneously exciting all of photocathode material layer 30 therebyto provide the low velocity flood beam of photoelectrons directed towardand through the storage screen 40 and onto 7 stances he preferred sinceno external lamps are required, theembodimentof Fig. 2 may .beadvantageous since the entire photocathode material layer 30 isinstantaneously illuminated during viewing and erasing rather than beingilluminatediby the writing spot exciting a raster as in the embodimentof 'Fig. .1. Thus, it is to be expected that with the embodiment of Fig.2, higher brightness and faster erasing will result since the charge perelement of cathode area emitted from the photocathode material layer 30.in the time T required to scan a raster would be if whereas .itwould beonly r z A in the embodiment of Fig. 1 with the scanned/raster where Aisthe area of the raster and .(a) the area ,of the light spot whichtraces the raster on the phosphor layergs thusprovidinga correspondingemission of photo electrons from the photocathode material layer 30 inraster form. I I

It will be readily seen that with both embodiments of my invention, themagnetic focusing coil 60 is axially polarized, as indicated in Fig. .1,thus providing amagnetic field with its lines of flux extending betweentarget electrode 27 and display screen 23 being essentially parallel tothe axis of the tube, thus uniformly focusing the photoelectrons fromthe photocathode material layer to emit high .velocityphotoelectronstoward said storage screen whcrebyia chargeimageaistformedonsaid'storage screen correspondingtto. said .:input. signals; and meansarranged .for at zotheri times .excitingasaid photocathode materiallayer to. emit' a uniform (flood beam of low' velocityphotoelectronstoward saidstorage screen whereby said low velocityphotoelectrons pass through sa1d storage screen. being modulated bythe.charge image facing said display screen andarrangedtto be excited byfluorescence of said phosphor layer; a perforate-charge storage screenpositioned intermediate said target elec- 'trode andtsaididisplayscreen; anelectron gun arranged to scan an electron beam over saidphosphor layer thereby causing fluorescence of the same; circuit con--onto and through the storage screen and onto the I display screen 3 andin turn providing a focuseddisplay image withthe resolution of priorstorage image converter tubes and with the brightness and .contrastcapabilities of prior signal-to-image, storage tubes. It will furtherbeseenthat with my improved storag cathode ray. tube construction, the writingelectron gun:

is disposed along the axis of the tube thereby eliminating thedistortion encounteredin prior signaleto-image storage tubes in whichthe writing gun is off-set with respect to the axis of'the tube. I

7 While I have described above the principles ofmy invention inconnection with specific apparatus; it is to be clearly understood thatthis description is made only by way of example and not as a limitation.to the scope of my invention.

What is claimed is:

1 A direct viewing storage cathode ray tube comprising: a phosphordisplay screen; a planar target .electrode spaced from said displayscreen and including ,a phosphor layer and a layer of photocathodematerial fs iss vsai di a sc nda r ge t be excited by fluorescence ofsaid phosphor layer; a perforate charge storage screen arrangedintermediate said target electrode and said display screen; electrongunmeans arranged toscanan electron beam over said phosphor layer therebycausing fluorescence of the same and exciting said p tqcat o m e l s tem h h ve i y pho electrons toward said storage screen; means forcausing said .photocathode material layer .to emit low velocityphotdelectrons toward said storage screen and magnetic focusing meansarranged to focus said photoelectrons onto said charge storage screenand to focus an electron image of the charge storage screen onto saiddisplay screen. i a

2. A direct viewing storagecathode ray tube comprising: ,a phosphordisplay screen; a planar target electrode spaced from said displayscreen and including a phosphor layer and a layer of photocathodematerial facing said display screen and arranged to be excited byfluorescence of said phosphor layer; a perforate charge storage screenpositioned intermediate said target electrode and said display screen;electron gun means including a signal input circuit adaptedto beconnected to a source of input signals, said electron gun means beingarranged at times to scan an electron beam responsive .to said inputsignals over said phosphor layer thereby causing fluorescence of thesame and exciting said hotocathode material layer nections arrangedduring first intervals to impress an input signal on said electron .gun.and 'to impress a first predetermined potential. on said targetelectrodeso that saidelectron gun scans said phosphorlayer withanelectron beam responsive to said input signal thereby exciting saidphotocathode material layer toernit high velocity photoelectrons towardsaidstorage screen to form a charge image thereon responsive to saidinput signal;

said circuit connections being arranged during second intervals todisconnect said input signal from said electron gun and to excite saidphotocathode material layer and impressanother predetermined potentialthereon so that said photocathode material layer .emits a'uniform' floodbeam of low velocity photoelectrons toward said storage screen wherebysaid low velocity photoelectrons pass through said storage screen beingmodulat'ed bythe charge image thereon to provide a visual image on saiddisplay screencorresponding to said charge image; and magnetic focusingmeansextending :between said target electrode and said display screenfor focusing said high and low velocity photoelectronsonto-said storagescreen and said low velocity photoelectronsronto said display screen. i

4. The combination of claim3 :in which said first predeterminedpotential .is highly negative with respect to the potential of saidstoragerscreen and said other predetermined potential is slightlynegative with respect to the potential of said storage screen. 1 I

'5. A direct viewing storage cathode ray tube comprising: a phosphordisplayscreen; apla nar t argetelectrodeflspaced from said displayscreen and including a phosphor layer and a layer of ,photocathodematerial facing said display screen and arranged to be excited byfluorescence of said .phosphorlayer; a perforate charge storage screenpositioned intermediate said target electrode and said display screen;an electron gun arranged to scan an electron beam over said phosphorlayer thereby causing fluorescence .of thesaine; circuit connectionsarranged during first intervals to impress an input signal on saidelectron gun .and to impress a predetermined potential on said targetelectrode so that said electron gun scans said phosphor layer with anelectron beam responsive tosaid input signal thereby excitingsaidphotocathode material layer to emit high. velocity photoelectronstoward said storage screen to form a charge image thereonresponsive tosaid input signal; said circuit connections being arranged during secondintervals to disconnect said input signal from said electron gun and toimpress another predetermined potential on said photocathode materiallayer so that said electron gun scans said phosphor layer with aluniformintensity electron beam whereby said photocathode material layer emits auniform flood beam of low velocity photoelectrons toward said storagescreen whereby said low velocity photoelectrons pass through saidstorage screen being modulated by the charge image thereon to provide avisual image on said display screen corresponding to said charge image;and magnetic focusing means extending between said target electrode andsaid display screen for focusing said high velocity photoelectrons ontosaid storage screen and said low velocity photoelectrons onto saiddisplay screen.

6. A direct viewing storage cathode ray tube comprising: an enclosingenvelope; a phosphor display screen at one end of said envelope; aplanar target electrode in said envelope spaced from said display screenand including a phosphor layer and a layer of photocathode materialfacing said display screen and arranged to be excited responsive tofluorescence of said phosphor layer; a charge storage screen in saidenvelope intermediate said target electrode and said display screen andincluding an insulator layer facing said photocathode material layer; anelectron gun in said envelope at the other end thereof arranged to scanan electron beam over said phosphor layer thereby causing fluorescencethereof; a signal input circuit; switching means arranged in a firstposition to connect said signal input circuit to said electron gun andto impress a potential on said photocathode material layer highlynegative with respect to the potential of said charge storage screen sothat said electron gun scans said phosphor layer with an electron beamresponsive to said input signal thereby exciting said photocathodematerial layer to emit high velocity photoelectrons toward said storagescreen forming a charge image on said insulator layer corresponding tosaid input signal; said switching means being arranged in a secondposition to disconnect said signal input circuit from said electron gunand to impress a potential on said photocathode material layer slightlynegative with respect to the potential of said charge storage screen sothat said electron gun scans said phosphor layer with a uniformintensity electron beam thereby exciting said photocathode materiallayer to emit a uniform flood beam of low velocity photoelectrons towardsaid storage screen which pass through said storage screen beingmodulated by the charge image thereon to provide a visual image on saiddisplay screen corresponding to said charge image; and an annularmagnetic focusing coil surrounding said envelope and extending betweensaid target electrode and said display screen for focusing said high andlow velocity photoelectrons onto said storage screen and said lowvelocity photoelectrons onto said display screen.

7. A direct viewing storage cathode ray tube comprising: an enclosingenvelope; a phosphor display screen at one end of said envelope; aplanar target electrode in said envelope spaced from said display screenand having a transparent film with a phosphor layer on one side thereofand a layer of photocathode material on the other side facing saiddisplay screen and arranged to be excited responsive to fluorescence ofsaid phosphor layer, said phosphor and photocathode material layersbeing electrically connected together; a charge storage screen in saidenvelope intermediate said target electrode and said display screen andhaving a fine mesh metal screen with secondary emissive insulatormaterial deposited thereon facing said photocathode material layer; asecondary electron collector electrode in said envelope intermediatesaid target electrode and said storage screen; an electron gun in saidenvelope at the other end thereof and including a cathode and a controlgrid, said electron gun being arranged to scan an electron beam oversaid phosphor layer; a signal input circuit, switching means arranged ina first position to connect said control grid to said signal lnputcircuit and to a source of grid bias potential and to connect saidphosphor and photocathode material layers to a source of potentialhighly negative with respect to the potential of said storage screen sothat said electron gun scans said phosphor layer with an electron beamresponsive to said input signal thereby exciting said photocathode layerto emit high velocity photoelectrons toward said storage screen forminga charge image on said insulator material corresponding to said inputsignal; said switching means being arranged in a second position thereofto connect said electron gun control grid to said cathode and saidphosphor and photocathode material layers to a source of potentialslightly negative with respect to said storage screen so that saidelectron gun scans said phosphor layer with a uniform intensity electronbeam thereby exciting said photocathode material layer to emit a uniformflood beam of low velocity photoelectrons toward said storage screenwhich pass through said storage screen being modulated by the chargeimage thereon to provide a visual image on said display screencorresponding to said charge image; and an annular magnetic focusingcoil surrounding said envelope and extending between said targetelectrode and said display screen for focusing said high and lowvelocity photoelectrons onto said storage screen and said low velocityphotoelectrons onto said display screen.

8. A direct viewing storage cathode ray tube comprising: an enclosingenvelope; a phosphor display screen at one end of said envelope; aplanar target electrode in said envelope spaced from said display screenand including a transparent phosphor layer and a layer of photocathodematerial facing said display screen and arranged to be excitedresponsive to fluorescence of said phosphor layer; a charge storagescreen in said envelope intermediate said target electrode and saiddisplay screen and including an insulator layer facing said photocathodematerial layer; an electron gun in said envelope at the other endthereof arranged to scan an electron beam over said phosphor layerthereby causing fluorescence thereof; a signal input circuit; switchingmeans arranged in a first position to connect said signal input circuitto said electron gun and to impress a potential on said photocathodematerial layer highly negative with respect to the potential of saidcharge storage screen so that said electron gun scans said phosphorlayer with an electron beam responsive to said input signal therebyexciting said photocathode material layer to emit high velocityphotoelectrons toward said storage screen forming a charge image on saidinsulator layer corresponding to said input signal; a flood light sourcedisposed outside of said envelope and arranged uniformly to illuminatethrough a transparent wall of said envelope the side of said phosphorlayer remote from said photocathode material layer; said switching meansbeing arranged in a second position to disconnect said signal inputcircuit from said electron gun and to impress a potential on saidphotocathode material layer slightly negative with respect to thepotential of said storage screen and to illuminate said flood lightsource whereby said photocathode material layer is excited to emit auniform flood beam of low velocity electrons toward said storage screenwhich pass through said storage screen being modulated by the chargeimage thereon to provide a visual image on said display screencorresponding to said charge image; and an annular magnetrc focusingcoil surrounding said envelope and extending between said targetelectrode and said display screen for focusing said high and lowvelocity photoelectrons onto said storage screen and said low velocityphotoelectrons onto said display screen.

OTHER REFERENCES Knoll: Storage Tubes and Their Basic Principles, JohnWiley and Sons, Inc., New York, 1952, pp. 78-81,

